Temperature-responsive system of the bridge circuit type



Oct. 16, 1951 M. F. PETERS 2,571,605

TEMPERATURE RESPONSIVE SYSTEM OF THE BRIDGE CIRCUIT TYPE Filed Aug. 16,1949 INVENTOR MELV/L LE F PETERS Jain/ Wm iY-i mm ATTORNEY-5 PatentedOct. 16, 1951 TEMPERATURE-RESPONSIVE SYSTEM OF THE BRIDGE CIRCUIT TYPEMelville F. Peters, East Orange, N. J., assignor to Petcar ResearchCorporation, Newark, N. J a corporation of New Jersey Application August16, 1949, Serial No. 110,647

1 Claim. 1

This invention relates broadly to devices and systems which areresponsive to an increase in the ambient temperature, to a predeterminedtemperature, to cause the operation of some indicating or control meansor other operating device, and more particularly the invention has to dowith such devices and systems which operate by reason of the change inelectrical conductivity of an element with increase in temperature. Suchdevices and systems are useful to eifect control operations, to indicatethat a predetermined temperature has been reached, or otherwise, and forpurposes of illustration only the invention will be described in thisapplication as a fire detecting system.

By this invention I have provided a fire detector system comprising abridge circuit which is normally in balance, thus providing a certainindication on a suitable indicating instrument, but which is unbalancedby the effect of rise of ambient temperature to a predetermined degreeto change the indication of the instrument to indicate the existence ofthe predetermined ambient temperature.

A system according to my invention is illustrated in the drawing anddescribed-in the specification of this application, it being understoodthat the same is only illustrative of the inven-- tion; which may takeall other forms within the scope of the claims to the invention.

In the drawings of this application,

Fig. 1 is a circuit diagram 01 a fire detecting system according to theinvention, and

Fig. 2 is a schematic circuit diagram illustrating the electricalrelations of various parts of the system of Fig. 1.

In the fire detector system illustrated in Fig. 1 of the drawing theletter A represents a temperature-responsive element comprising twospaced, parallel, electrically conductive wires 2, 4, each of which hasresistance r, and which are spacedapart and surrounded by beads 6 formedof thermistor material, by which is meant material which has asubstantially greater electrical resistance at normal temperatures thanit has at a predetermined higher temperature. This temperatureresponsive element is fully disclosed in my co-pending application,Serial No. 90,881 and may, if desired, be manufactured by the methoddisclosed in my co-pending application Serial No. 2,344 now Patent No.2,495,867. The two conductors 2, 4 of element A are connected at theirone ends through resistor 8, which has resistance r1, one side ofwhich,e. g. at its connection with conductor, 2, is grounded at I0. At

the other end of element A conductor 2 is connected in series with aresistance l2 having value R, while conductor 4 is connected in serieswith resistance I4, I 6 having values R and R1, respectively, theresistance R1 of resistor l6 being large with respect to the resistanceR of resistors 12, I 4. A source of electrical energy I8 which may be A.C. or D. C. is connected between ground and the connection betweenresistors 12, It. The connection between conductor 2 and resistor l2,which is designated X, is connected by leads I 8, 20 to the grid of avacuum tube 22 and is also connected through resistance 24 to thecathode of the tube. The connection between conductor 4 and resistorsl4, it, which is designated Y, is connected through lead 28 to thecathode of tube 22 at a point between the cathode and resistor 24. Asource of electrical energy 28 is connected between the anode of tube 22and an indicator on signal device 30, the second terminal of which isconnected to the cathode of tube 22.

The described elements constitute a bridge circuit which is shownschematically in Fig. 2. Each of the resistances has such a value thatthe relation of the resistance, reference being made to the numeralsidentifying them, is:

It will be seen that so long as this relation of resistance ismaintained the bridge will be balanced and there will be no potentialdifference between the equi-potential points X and Y which are,respectively, the point of connection between resistor 2 and resistorI2, and the point of connection between resistors 4, 8 and resistors l4,it. As resistor 24 is connected across these two points there will be novoltage drop across the resistor so long as the balanced bridgecondition is maintained and, under this condition, the

the ambient temperature of the monitored space increases to apredetermined temperature the resistance of the beads, at the locale ofthe increased temperature, will decrease to a predetermined lower value,thus establishing electrical connection between the conductors at thatpoint. For purposes of illustrating the invention it will be assumedthat the rise in temperature occurs at the bead marked Z in Fig. 1. Whenthis occurs the resistor 8, which is in parallel with the bead or beadsin region Z. will cause the current fiow in 8 and Z to be inverselyproportional to their resistance. This in substance is the same aschanging the resistance of 8 which unbalances the bridge circuit andcauses a potential difference between X and Y, and consequently acrossresistor 24. This actuates the grid of tube 22, completin the signalcircuit through the tube and operating the signal. Upon reduction of thetemperature at Z, the resistance of Z increases until the current flowthrough Z may be neglected compared to the current flow through 8 whichagain balances the circuit. With balancing of the circuit the potentialdifferent between X and Y is reduced to zero and the current 'fiowthrough resistor 2Q to zero, so that the potential difierence betweenthe grid and cathode of the tube is zero, thus opening the signalcircuit.

While I have illustrated and described one form of the invention, itwill be evident to those skilled in the art that other forms, as well asmodification thereof, may be made without departing in any way from thespirit or scope of the invention for the limits of which reference mustbe made to the attached claim.

What is claimed is:

A temperature-responsive system comprising an element adapted to bepositioned in a space subject to variations of temperature to indicate apredetermined abnormal high temperature therein, said element comprisingtwo spaced conductors of electricity each having a known finiteresistance '1' and being spaced apart by material having such a highresistance at normal temperatures that it effectively insulates the two4 conductors and having such a low resistance at a pre-determined highertemperature that it forms a conductor of electricity between the twoconductors, a known resistance 11 connected in series with the first ofsaid conductors to form one arm of a balanced bridge circuit the sec--and arm of which is formed by the resistance of the second conductor andthe connection between said arms being grounded, the third arm of saidbridge being formed by a known resistance R in series with the secondconductor and the fourth arm being formed by a resistance R+Rl greaterthan resistance R connected in series with the first conductor, theconnection between said third and fourth arms being grounded through asource of electric energy and the resistances of the arms of the bridgewhen in balance having the following relation:

i r+r, R+R

when only said element is subjected or responsive to temperaturevariation, a circuit including an operatin device, a source of electricenergy and means controlled by voltage across the balanced points ofsaid bridge for maintaining said circuit de-energized when the bridge isbalanced and for energizing the circuit when the bridge is unbalanced.

mm F. PETERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Great Britain Jan. 26, 1948

